RealityView camera feed not shown

I have two RealityView: ParentView and When click the button in ParentView, ChildView will be shown as full screen cover, but the camera feed in ChildView will not be shown, only black screen.

If I show ChildView directly, it works with camera feed.

Please help me on this issue? Thanks.

import RealityKit
import SwiftUI

struct ParentView: View{
 @State private var showIt = false
 var body: some View{
 ZStack{
 RealityView{content in
 content.camera = .virtual
 let box = ModelEntity(mesh: MeshResource.generateSphere(radius: 0.2),materials: [createSimpleMaterial(color: .red)])
 content.add(box)

 
 }
 Button("Click here"){
 showIt = true
 }
 }
 .fullScreenCover(isPresented: $showIt){
 
 ChildView()
 
 .overlay(
 Button("Close"){
 showIt = false
 }.padding(20),
 alignment: .bottomLeading
 )
 }
 .ignoresSafeArea(.all)
 }
}

import ARKit
import RealityKit
import SwiftUI
struct ChildView: View{
 var body: some View{
 RealityView{content in 
 content.camera = .spatialTracking
 }

 }
}